The mouse oocyte is a non-proliferating, differentiated cell. Following fertilization and the first round of DNA replication, the 2-cell stage blastomeres, which contain the first zygotic nuclei, are totipotent. This remarkable transformation probably entails reprogramming the pattern of gene expression. The molecular basis for this reprogramming is poorly understood. Specific Aim 1 will (1) examine the role of DNA replication in this reprogramming and will identify genes by mRNA differential display whose expression is coupled to the first round of DNA replication, (2) determine if a competition between maternally- derived histones and transcription factors controls the timing for the onset of zygotic transcription and the spectrum of genes that are expressed by expanding the endogenous histone pool via microinjection and then assaying for the expression of specific markers of zygotic gene activation and (3) assess the role of chromatin structure in the reprogramming of gene expression. Following genome activation, the blastomeres are totipotent up to the 8-cell stage. A spatial reprogramming in the pattern of gene expression in the morula correlates with cell specification and differentiation of the outer cells into trophectoderm and the maintenance of totipotency of the inner cell mass cells that give rise to the embryo proper. The molecular basis that underlies this spatial change in the pattern of gene expression is also poorly understood. Specific Aim 2 will use an RT-PCR based method coupled with mRNA differential display to identify genes whose expression is spatially restricted in the blastocyst. These genes, and genes whose spatial pattern of expression is already known to be restricted in the blastocyst, will then be used as markers to determine when these changes in expression are first detected and the role of cell-cell contract in their spatial expression patterns. Last, changes in TAT-box and enhancer utilization will be examined using a series of reporter transgenes driven by different combinations of TATA-box and enhancer elements to determine if changes in promoter utilization may contribute to the spatial differences in gene expression. By providing new information regarding gene expression in the preimplantation embryo, these studies will expand our basic understanding of how the preimplantation embryo executes its essential functions of cell proliferation and differentiation; failure to execute successfully this program may contribute to the high degree of embryo loss that occurs prior to implantation.